The present invention relates to a method for forming a polyimide film by chemical vapor deposition. The resultant polyimide film is useful as an insulating, passivating or moistureproof protective film of a semiconductor element, or an orientation control film of a liquid crystal display element.
A known process for preparing a polyimide film, a so-called wet process comprises polymerizing a diamino compound with a tetracarboxylic acid or its derivative in an organic polar solvent to prepare a polyamidc acid varnish, applying the varnish on a substrate and heating the varnish to dry and to convert into polyimide. This process tends to give a thin film having pinholes, so that the preparation of a high quality film is difficult. This tendency is particularly marked, when a thin film having a thickness of lower than 1 .mu.m is prepared. Further, the process has another disadvantage in that the obtained film tends to include impurities due to the solvent used.
U.S. Pat. No. 4,625,867 discloses a process for preparing a polyimide film which comprises simultaneously evaporating two monomers for a polyimide such as pyromellitic acid dianhydride and 4,4'-diaminodiphenyl ether with a suitable mol ratio to deposit them on a substrate and polymerizing them thereon by heating for overcoming the above disadvantages of the wet process. This process requires continuous measurement of the temperature of the monomers during the heating and thereby controlling it exactly in order to prepare a uniform and high strength film, because an exact control of the evaporation rate of each of the monomers is needed to obtain a high molecular weight polyimide which can give a uniform and high strength film. However, the exact control of the temperature, in other words, the evaporation rate of the two monomers is very difficult in the process, so that the preparation of a high molecular weight polyimide film is very difficult. More precisely, if the molar ratio of the monomers to be reacted is deviated from its stoichiometric value only by 10%, the obtained polyimide film will have a remarkably low molecular weight. For example, when a diamine is reacted with a tetracarboxylic anhydride with an exact molar ratio of 1:1, the resultant polyimide has a molecular weight of 20,000 to 50,000, while when a diamine is reacted with a tetracarboxylic anhydride with a molar ratio of 10:9 or 9:10, the resultant polyimide has a molecular weight of about 3800 at the most.
As reported, for example, in M. L. WALLACH "Structure-Property Relations of Polyimide Films" (JOURNAL OF POLYMER SCIENCE: PART A-2, VOL. 6, 953-960, (1968)), it is well known that the strength of a polyimide film lowers with lowering molecular weight thereof.